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Pros

Pro

Free unlimited private repositories for small teams

Bitbucket offers unlimited private repositories for free, as long as the number of members in a team is not larger than 5. BitBucket does not use GitHub's pricing plan, instead of charging users for each private repo, it charges teams per number of team members. It is free for up to 5 people.

Pro

Supports both Mercurial and Git

Has full support for both Mercurial and Git VCS.

Pro

Native application for both Mac and Windows

Atlassian, the company behind BitBucket is also behind SourceTree, a free application for Windows and Mac wich works as a client for both Git and Mercurial which can be connected to BitBucket and other code hosting services.

Cons

Con

Only 5 users are free

Con

Integration with other tools is not as good as github

Most of the 3rd party tools ( bug trackers, CI servers, chat servers, etc) always integrate with GitHub first, and either don't integrate with Bitbucket at all or do sub par job in it. For example there are 3 separate projects to allow building and verifying pull request submissions on Jenkins CI server and there is only one viable one for BitBucket and that one is really buggy and lacking many features.

The only exception to this rule is Atlassian products like Jira.

Con

The UI seems old and not very polished

While GitHub's UI is extremely simple to understand and very polished, BitBucket lacks a bit on this category. With a design that seems old and not as pleasant to look at.